More efficient underwater PV cells with wide-band gap semiconductors

U.S. researchers have conducted detailed balance calculations to show the ultimate potential of underwater solar cells. According to their findings, the devices could theoretically produce useful power at up to 65% efficiency in clear waters. However, this would only be possible by using wide-band-gap semiconductors, which have not been considered for solar cells used for land-based applications because their band gaps are too large.

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A research team from New York University is trying to assess the potential efficiency limits of underwater solar cells.

In Efficiency Limits of Underwater Solar Cells, published in Joule, the scientists stated that such devices could generate useful power in deep waters. But they noted that wider band-gap semiconductors should be used for the cells instead of the narrow band-gap materials that are used for traditional crystalline PV devices.

Previous results

“Previous attempts to use underwater solar cells to run autonomous systems have had limited success due to the use of solar cells made from silicon (Si) or amorphous Si (a-Si), which have band gaps of 1.11 and 1.8 electronvolt (eV), respectively, and are optimized to function on land,” the researchers said.

Other studies have shown that indium gallium phosphide (InGaP) solar cells, which have a band gap of around 1.8 eV, could be more efficient in producing power at depths as low as nine meters below sea level. However, the devices are still too expensive, in spite of recent progress on cost reductions.

As an alternative, the researchers have proposed the use of organic and inorganic wide-band-gap semiconductors, which are not being considered for solar cells at the moment because their band gaps are too large for land-based applications.

Maximum theoretical efficiency

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Crystalline solar cells based on narrow gap semiconductors have a maximum theoretical efficiency of 34%, which is the so-called Shockley-Queisser limit. Indoor solar cells based on organic materials can reach maximum theoretical efficiencies of around 60% when illuminated by light-emitting diodes (LEDs) and approximately 67% when illuminated by sodium discharge lamps, the researchers stated.

As for solar cells relying on wide band-gap semiconductors operating underwater, the scientists estimated that their maximum theoretical efficiency stands between around 55% at two meters to more than 63% at 50 meters. “The large increase of the solar cell efficiency beyond the Shockley-Queisser limit, even in shallow waters (two meters), is due to the narrowing of the transmitted solar spectrum reaching the solar cell,” they explained. “An additional boost in efficiency can be achieved when the solar cells are operated in cold waters.”

The research team said that the optimum band gap of the cell absorber shifts from around 1.8 eV when operated at two meters to approximately 2.4 eV at 50 meters, with a band-gap plateau at about 2.1 eV between four and 20 meters. “We also show that the optimum band-gap values are more or less independent of which waters the solar cell is deployed in, which is highly beneficial from a design perspective, as the solar cells would not have to be tailored to specific waters but rather to specific operating depths,” they said.

Suitable semiconductors

The researchers noted several direct inorganic wide-band-gap semiconductors that could be explored for applications in underwater solar cells. They include hydrogenated amorphous silicon, semiconductors such as copper peroxide (CuO2) and zinc telluride (ZnTe), as well as III-V semiconductors like aluminum gallium arsenide (AlGaAs), indium gallium phosphide (InGaP), and gallium arsenide phosphide (GaAsP).

They added that organic wide-band-gap semiconductors such as rubrene, pentacene and p-phenylene vinylene derivatives could be good candidates for producing such cells. “With the recent development of replacing fullerenes with non-fullerene acceptors to achieve both greater performing organic solar cells and improved device stability, a number of new wide-band-gap semiconductor donor materials have been developed, yielding higher efficiencies than traditional systems that were paired with fullerene derivatives,” the scientists said.

“Since wide-band-gap semiconductors are not conventionally desired for outdoor solar harvesting, the large library of inorganic and organic wide-band-gap semiconductors, currently not considered for land-based solar cells, could potentially be used as absorbers in high-efficiency underwater solar cells,” they concluded.

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Emiliano Bellini

Emiliano joined pv magazine in March 2017. He has been reporting on solar and renewable energy since 2009.

“Crystalline solar cells based on narrow gap semiconductors have a maximum theoretical efficiency of 34%, which is the so-called Shockley-Queisser limit.” That is in sunlight which is many visible (and some not visible) light frequencies, i.e. a broad band of light frequencies.

“Indoor solar cells based on organic materials can reach maximum theoretical efficiencies of around 60% when illuminated by light-emitting diodes (LEDs) and approximately 67% when illuminated by sodium discharge lamps, the researchers stated.” LEDs and sodium discharge lamps output much narrower band of light frequencies.

This is a complete apples to oranges comparison. Use a source of light which is narrower and better matched to silicon PV’s bandgap and the theoretical efficiency will be much higher. Conversely, use sunlight as your light source for “indoor solar cells based on organic materials” and your efficiency will be much lower.

‘ “The large increase of the solar cell efficiency beyond the Shockley-Queisser limit, even in shallow waters (two meters), is due to the narrowing of the transmitted solar spectrum reaching the solar cell,” they explained.’ Right. The two meters of water is absorbing much of the rest of the solar spectrum, so you are incurring a significant loss of efficiency before you even reach the PV cell. The goal is to convert sunlight to electricity. Let’s include the available sunlight at the surface of the water in this efficiency calculation for a realistic comparison with land based silicon PV efficiency.

‘the scientists estimated that their maximum theoretical efficiency stands between around 55% at two meters to more than 63% at 50 meters. ‘ You know you’ve lost most of your sunlight at about 10 meters, roughly 30 feet, right?

Most of the electromagnetic spectrum (which includes light) is severely attenuated in water. In seawater there are only two windows where absorption of light is reduced: ELF/VLF (extra and very low frequencies) not applicable here, and blue-green light.

One last point: Biofouling! Algae will grow on your panels in fresh water. Algae, barnacles, anemones, tube worms, etc will grow on them in salt water. You’d be better off putting a light filter over your land based PV panel so you can claim high efficiency after the filter, a light filter matching the band-gap of your silicon PV cells. Nobody will do that, because your efficiency of generating electricity from sunlight would be reduced …and that is the whole point of having solar pv.

Get these labs dudes out in the field installing solar pv panels for a while.

Mr Bellini, You put up some really great PV news articles. I’m a fan. This isn’t one of them. It needs a common sense reality check. No offense intended, just saying. Either I’m missing something or this is an article lacking in contextual reference to reality. Don’t let them get away with that. Thank you, mike

Dear Mark, Hi Mike, Thanks for your comment. I don’t think we will see underwater solar cells being widely adopted in submarine devices any time soon, but I am convinced the research we reported on is pretty interesting. I really like to read (and write) on topics like these, regardless of the devices’ real chances to exist in the future. Many scientific researches are focused in something “lacking in contextual reference to reality,” let’s say this one of the ideal targets of science. In ten or twenty years, we will see if these guys were right or not about the submarine PV devices. In the meantime, I would let them get away with that…

Can wide band-gap semiconductors be applied to ( not so ? ) deep sea pumped storage as Ballast for each Offshore Wind Turbine as autonomous Units, & for enhanced if not also dual generation Cheers Jeff M. Weeks Researcher

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